In the U.S. patent application Ser. No. 299,979 filed Sept. 8, 1981 (assigned to the assignee of the invention disclosed herein and now U.S. Pat. No. 4,501,064 granted Feb. 26, 1985) there is disclosed an invention for a machine for placing chip type electronic components on a printed circuit board at a very high rate of speed. This machine requires an adhesive dispenser for dispensing a non-conductive adhesive onto a printed circuit board between the conductive lands on the board. The chip is subsequently placed onto the adhesive to make the chip adhere to the board between the conductive lands. The board can be later processed through wave soldering apparatus to make the final electrical bond between the component and the conductive lands on the board.
In investigating available apparatus for dispensing the adhesive it became apparent that such an apparatus must accurately dispense very small amounts of a thick paste-like material at a rate of at least four times a second with sufficient reliability to be incorporated into an automatic system without causing any potential system down-time to exceed the time necessary to repair any other operating parts of the system. All of these requirements additionally must be accomplished by using the mechanical and electrical resources available on the component placement machine.
There are generally two methods of applying or depositing drops of fluid at specified locations; the transfer method and by the direct application. An example of the transfer method is a rubber ink stamp where the fluid is picked up by an applicator and deposited where desired. Available apparatus using this method of applying adhesives to a printed circuit board are undesirable since they are not general purpose devices, must be especially designed and built for specific application and as a rule are not programmable.
Direct application of a fluid to a substrate is accomplished by inducing the fluid to flow from a reservoir to an applicator having an orifice that deposits the fluid. The fluid is caused to flow through a conduit from the reservoir to an applicator by some type of displacement means caused either by compressed air (pneumatic) or mechanical reduction of the reservoir volume to move the fluid.
There are two ways of using the pneumatic displacement method to induce fluid flow. The simple way is to apply pressure to a reservoir when the flow of material is desired and relieve pressure when the required amount is obtained from the applicator. When used with relative thin fluids, these devices are sometimes rigged so that a mild vacuum (termed "suckback") is applied to the reservoir when the pressure is removed in order to prevent the fluid from escaping spuriously (drooling) from the applicator under the effects of gravity or capillary action. In these devices, blunt hollow needles are commonly used as the outlet orifice of the applicator.
The second method of using pneumatics for dispensing fluids involves keeping the reservoir pressurized constantly whenever the system is in use. A valve is interposed between the reservoir and the applicator to control the release of the fluid. These valves can be manually operated or actuated by electrical, pneumatic, hydraulic or various mechanical linkages. In these devices the fluid must travel through the inner workings of the valve and when the fluid is an adhesive that can setup or harden, the valve becomes non-functional until cleaned.
With regard to using a mechanical means to reduce the reservoir volume to move the fluid, this can be done by either using a plunger acting on the fluid or by squeezing the reservoir.
None of the prior art devices of the nature described provide for metering a small amount of paste-like adhesive with a degree of repeatability and reliability within a specified limited time interval for application to a chip type component placement machine. The devices mentioned utilized a hollow needle as the outlet of the applicator. The object is to have the adhesive protrude beyond the end of the needle and bring the needle outlet adjacent the printed board substrate to apply the small amount of adhesive to the desired location. In theory, the protruding material will separate from the main mass of adhesive at the plane of the outlet and remain on the substrate when the needle is removed. In practice, however, the contact of the needle to the substrate tends to pack the fluid back into the needle orifice and upon removal of needle, the fluid separates up inside the orifice leaving a void at the end of the needle which may not be completely filled in the next cycle, resulting in no deposit occurring at that time. Conversely, upon contact between the needle and substrate, a sufficient amount of material may be pushed back into the needle orifice so that enough grip between the adhesive and the ID of needle occurs to overcome the cohesion and surface tension of the adhesive to the substrate so that a deposit of adhesive on the substrate does not take place. These problems can be partially overcome when the material droplet is fairly large compared to the bore diameter of the needle. However, in the application of small metered amounts of adhesives, the droplet size is approximately the same as the size of the needle and thus a hollow needle applicator is insufficient for this application.